How can I change smoke colour?
Select the smoke flow object and in the physics tab, click the option that says "Smoke Colour" a colour wheel will appear. Using the colour wheel and the lighten darken option to its right, pick your desired colour. Things I did: 


added a white plane behind the simulation; it's easier to see. 
moved the camera to a better angle
changed the smoke colour in the smoke flow object, not the domain.